At a meeting held in Amsterdam at the time of the tenth International Congress of Philosophy, it was suggested to one of us that he write a
monograph on combinatory logic as one of a projected series of small
volumes on logic and the foundations of mathematics. The proposers of
this idea could hardly have had any conception of the amount and
complexity of research in that field which had never been published;
what they seemed to have in mind was an expository work, which would
make available in readable and systematic form the extent of our present
knowledge about the subject. Nevertheless the suggestion made at the
meeting, and the ensuing discussion, sowed an acorn in the mind of one
of us; and from that acorn the present sapling has grown.